This invention relates to a multilayered container, and more specifically, to a multilayered container having excellent delamination resistance, transparency, gas-barrier property and mechanical strength.
In recent years, hollow containers produced from thermoplastic resins have found widespread use as containers for cosmetics, foods, beverages, etc. because of their various advantages such as light weight and safety against bursting. Pariticularly, hollow containers of polyethylene terephthalate have been rapidly developed as a result of advances in the biaxial stretch-below molding techniques.
Biaxially oriented containers of thermoplastic polyester resins, mainly polyethylene terephthalate, do not necessarily have perfect properties for packaging purposes. When, for example, foods and beverages which require a high level of gas barrier property are held in such containers, the flavor of the contents will be spoiled because these resins do not possess sufficient oxygen and carbon dioxide gas barrier properties.
In an attempt to remove this defect, there was proposed a multilayered container obtained by injection-molding a thermoplastic polyester resin and then a m-xylylene group-containing polyamide resin (to be abbreviated as MX nylon) as a thermoplastic gas-barrier resin from separate injection cylinders into a single mold to form a three-layerd parison composed of an inside layer of the thermoplastic polyester resin and an outside layer of the thermoplastic polyester resin and a core layer of the MX nylon, and biaxially stretch-below molding the parison (Japanese Laid-Open Patent Publications Nos. 128516/1982 and 128520/1982; and the corresponding U.S. Pat. No. 4,535,901). If in this method, the amount of the MX nylon to be injected is decreased in an attempt to reduce the thickness of the core layer, the core layer is formed only partly, and the resulting container has insufficient gas-barrier property.
The present inventors invented a method improved over the above method, in which a thermoplastic polyester resin, MX nylon and the thermoplastic polyester resin are injected sequentially in this order to form a five-layered structure with three layers of the thermoplastic polyester resin and two layers of MX nylon being laminated alternatively, and as a result, in spite of the two layers of MX nylon, the amount of MX nylon injected can be decreased from that in the prior art (Japanese Laid-Open Patent Publication No. 240409/11980 and corresponding U.S. patent application Ser. No. 87969 and European Patent Publication No. 161625; Japanese Laid-Open Patent Publication No. 108542/1986 and corresponding U.S. patent application Ser. No. 793095 and European Patent Publication No. 180,191).
There was also proposed a method in which a resin forming an inside and an outside layer is first injected, and then a resin A forming an inside layer and an outside layer and a resin forming a core layer are simultaneously injected thereby reducing the thickness of the core layer (Japanese Patent Publication No. 16326/1985 and corresponding U.S. Pat. No. 4,174,413). This method can be applied when the core layer is made of a thermoplastic gas-barrier resin, but gives a three-layered structure in which the core layer is deviated toward one side from the center.